The present invention relates to a device for igniting an air-fuel mixture in an internal combustion engine using a high-frequency power source according to the definition of the species in the main claim.
Igniting such an air-fuel mixture using a so-called spark plug is customary in internal combustion engines for motor vehicles. In these ignition systems used today, the spark plug is inductively supplied with an adequately high voltage via an ignition coil so that an ignition spark is generated at the end of the spark plug inside the combustion chamber of the IC engine, triggering the combustion of the air-fuel mixture.
Voltages of more than thirty kilovolt may occur during operation of these conventional spark plugs, the combustion process producing residues, such as soot, oil, or carbon as well as ash from fuel and oil, which are electrically conductive under certain thermal conditions. However, at these high voltages, breakdowns must not occur at the spark plug insulator, so that the electrical resistance of the insulator should not change during the service life of the spark plug, even at the high temperatures occurring.
An ignition device in which the ignition of such an air-fuel mixture in an IC engine of a motor vehicle is performed using a coaxial line resonator is known from DE 198 52 652 A1 for example. The ignition coil is replaced here by a sufficiently powerful microwave source, a combination of a high-frequency generator and an amplifier, for example. In a geometrically optimized coaxial line resonator, the field intensity required for the ignition comes about at the open end of the plug-like line resonator and the voltage sparkover generates an ignitable plasma link between the electrodes of the plug.
Such a high-frequency ignition is also described in the article entitled “Investigation of a Radio Frequency Plasma Ignitor for Possible Internal Combustion Engine Use” published in “SAE-Paper 970071.” In this high-frequency ignition or microwave ignition, too, high voltage is generated via a low-resistance supply at the so-called hot end of a λ/4 line of an HF line resonator, without the ususal ignition coil.